As a bonding wire for a semiconductor device which connects electrodes on a semiconductor device and external leads (hereinafter referred to as a “bonding wire”), a thin wire with a wire diameter of about 15 to 50 μm is mainly used today. For a bonding method with bonding wire, there is generally used a thermal compressive bonding technique with the aid of ultrasound, in which a general bonding device, a capillary tool used for bonding by passing a bonding wire therethrough, and the like are used. A bonding process of a bonding wire is carried out by heating and melting a tip of wire by arc heat input to form a ball (FAB: free air ball) through surface tension; crimp-bonding the ball part onto an electrode of the semiconductor device heated within a range of 150 to 300° C. (hereinafter referred to as “ball bonding”); forming a loop; and finally crimp-bonding a wire part onto an electrode of the external lead (hereinafter referred to as “wedge bonding”). For the electrode on the semiconductor device as an object to be bonded with the bonding wire, there is used an electrode structure in which a film of alloy mainly containing Al is formed on a Si substrate. For the electrode of the external lead, there is used an electrode structure with Ag plating or Pd plating.
Although Au has been mainly used for a material of the bonding wire, a replacement of Au with Cu is being conducted mainly in LSI use. On the background of recent proliferation of electric vehicles and hybrid vehicles, needs for replacing Au with Cu are increasing also in on-vehicle device use.
As for a Cu bonding wire, there has been proposed a wire using high-purity Cu (purity: 99.99% by mass or more) (for example, Patent Literature 1). Cu has the drawback of being more susceptible to oxidation than Au and has problems in that bonding reliability, ball formability and wedge bondability are inferior. As a method for preventing surface oxidation of a Cu bonding wire, there has been proposed a structure in which a surface of Cu core material is coated with a metal such as Au, Ag, Pt, Pd, Ni, Co, Cr and Ti (Patent Literature 2). There has been also proposed a structure in which a surface of Cu core material is coated with Pd and a surface thereof is coated with Au, Ag, Cu or an alloy thereof (Patent Literature 3).